Reversible converter

ABSTRACT

A reversible converter for feeding a DC load such as a motor from an AC network has a current regulator which is supplied with a signal representing the actual value of the direct current and a desired value. The regulator is connected both to an emitter for delivering control pulses to the rectifier with a control angle which is dependent on the output voltage of the current regulator, and to a device for switching the polarity of the converter depending on the polarity of the desired current value, which causes a retarding of the converter towards inverter operation before shifting of the polarity of the converter, unblocking and advancing towards rectifier operation. A member which senses the voltage over the DC load is connected to the current regulator when the polarity of the inverter is shifted and delivers to it a signal corresponding to a control angle of the control pulses such that the sum of the inner direct voltage of the converter and the voltage over the DC load is slightly less than zero.

Hermansson et al.

15] 3,684,945 51 Aug. 15, 1972 [54] REVERSIBLE CONVERTER [72] Inventors:Bo Hermansson; Curt Hansson, both of Vasteras, Sweden Assignee: AllmannaSwenska Elektriska Aktiebolaget, Vasteras, Sweden Filed: Feb. 8, 1971Appl. No.: 113,657

[30] Foreign Application Priority Data [56] References Cited UNITEDSTATES PATENTS 12/1970 Maynard ..321/13 11/1970 Grafet a1 ..321/1312/1969 Wilkerson ..321/5 Primary Examiner-William H. Beha, Jr.Attorney-Jennings Bailey, Jr.

[5 7] ABSTRACT A reversible converter for feeding a DC load such as amotor from an AC network has a current regulator which is supplied witha signal representing the actual value of the direct current and adesired value. The regulator is connected both to an emitter fordelivering control pulses to the rectifier with a control angle which isdependent on the output voltage of the cur rent regulator, and to adevice for switching the polarity of the converter depending on thepolarity of the desired current value, which causes a retarding of theconverter towards inverter operation before shifting of the polarity ofthe converter, unblocking and advancing towards rectifier operation. Amember which senses the voltage over the DC load is connected to thecurrent regulator when the polarity of the inverter is shifted anddelivers to it a signal corresponding to a control angle of the controlpulses such that the sum of the inner direct voltage of the converterand the voltage over the DC load is slightly less than zero.

5 Claims, 6 Drawing Figures PATENTEnAus 15 I972 3 684. 945

sum 1 0F 2 1 5 E Ea M rg S m4 744 vi/us MA? I 7 5747/: 195447 MEANS [RTEN TC R BO HERMAN SSON BY CURT HANSSON REVERSIBLE CONVERTER BACKGROUND OFTHE INVENTION the input side of which are connected the instantaneous orthe actual value of the direct current of the converter and an emitterfor setting a desired DC value in the converter. The current regulatoris connected to a control pulse generator for the valves of theconverter, which control pulse generator delivers control pulses with acontrol angle which is dependent on the output voltage of the currentregulator. Further, the converter comprises a means for shifting thepolarity of the converter in relation to the load, the switching takingplace in dependence on the polarity of said desired DC value so that thepolarity of the converter corresponds to the polarity of the desired DCvalue.

Such means for shifting polarity may consist of a proper pole reverser,for example in the form of a contact switch or a valve switcher. Anotherpossibility is to make the converter of two reverse-parallelconnected(cross-connected) converters which are controlled so that one or theother is always blocked so that circulating currents are preventedbetween the two converters.

The current regulator is normally constructed as a control amplifierwhich is dependent on the difference between the real value and thedesired value of the current and emits an output voltage whichdetermines the control angle of the valves in relation to theircommutation voltages. The desired value may, for example, be derivedfrom the desired power of the DC load or, if this is a DC motor, fromthe desired speed of this mo tor. An order for increased speed will thuscause an increase in the desired value of the current and said controlangle is then reduced so that the direct voltage, and thus the directcurrent, of the converter increase until the desired speed has beenachieved.

An order for decreased motor speed or reversal of the motor causes thedesired value of the speed to be less than the actual value, so that thedesired value of the current changes sign and the converter is retardedtowards inverter operation, that is, the control angle of the valves isincreased so that the direct current rapidly drops to zero. Theconverter can then be blocked and its polarity is shifted, after whichthe converter is unblocked in a control state corresponding to inverteroperation. When the converter is then again advanced towards rectifieroperation, that is, the control angle is decreased, its internal directvoltage (still in the inverter area) will decrease until it is less thanthe armature voltage of the motor so that power is fed back from themotor to the converter and the speed of the motor drops rapidly.

When the motor is to be reversed the converter will be continuouslyadvanced towards rectifier operation as the motor speed drops and whenthis has become zero the converter transfers to rectifier operation sothat the power is fed from the converter to the motor and its speedincreases in the desired direction until the desired value is reached.

However, if the speed is only to be reduced to a lower value, passagebeyond the lower speed will cause the desired value of the current toshift polarity. The converter will then be retarded towards inverteroperation again so that it is again blocked, its polarity shifts againand the converter is unblocked and advances towards rectifier operationuntil its direct current cor responds to the desired value dictated bythe speed regulator.

In both cases the process is controlled completely automatically by thecurrent regulator and the pole shifter and it is obvious that thisshould happen as rapidly as possible. It is also equally obvious thatthe actual speed of alteration of the DC current and the motor speed isdictated by the inherent capacity of the various components so that thespeed of alteration of said magnitude is maximized in advance.

SUMMARY OF THE INVENTION All that remains to be shortened, therefore, isthe dead time between the various switching steps and the presentinvention relates to shortening the time from unblocking the converteruntil the moment when the converter again becomes current-carrying.According to the invention this is done by providing the converter witha member to sense the voltage over the DC load which member is connectedto the input side of the current regulator when the converter shiftspolarity. This member gives the current regulator an additional signalof a magnitude corresponding to such a control angle for the controlpulses that the inner direct voltage of the converter has such a valuethat the sum of this inner direct voltage and the voltage over the DCload, calculated in the conducting direction of the converter (totalvoltage) is less than zero by a certain small quantity. The magnitude ofthis quantity should substantially correspond to the amplitude of theripple in the direct voltage of the converter, that is, the amplitude ofthe alternating voltage superimposed on the direct voltage.

The consequence is that the resultant total voltage over the converterimmediately after unblocking is negative, so that no current will flowin the moment of unblocking, but this voltage is rather low and, as soonas the converter starts to advance towards rectifier operation, thecurrent starts to flow in the circuit. In practice the control angle isadjusted so that the ripple of the converter voltage causes an extremelylow pulsing current to flow through the circuit even during theunblocking although the average value of the total voltage is negative.

In the previously known constructions the converter is unblocked at fullinverter operation and the current cannot start to flow until theconverter has been advanced so far towards rectifier operation that itsvoltage has passed the value of the voltage over the load. It is thistime, that is, the time taken for advancing from full inverter operationto the point where the converter voltage has been adjusted to the loadvoltage, which is eliminated according to the invention. During thistime the load is, so to say, left to itself because of the interruptionin the current and, for example, the motor speed can alter arbitrarily,which gives rise to discontinuity in the control. This discontinuity iseliminated or considerably limited according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be furtherdescribed with reference to the accompanying drawings where FIGS. 1a, b,c, d show different stages in the switchover and FIG. 2 shows thecorresponding control angles, whereas FIGS. 3-5 show connectionsaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 14 and 2a show thestationary operating conditions. The converter S drives the motor M. Thecurrent direction is indicated by the arrow in FIG. la. The converter isdriven with a control angle 01,, which is derived by means of a controlfunction voltage E, from the feeding alternating current network and acontrol voltage E, from the current regulator so that the convertervoltage obtained is proportional to E The armature voltage of the motor,which is opposite to and lower than the converter voltage, is indicatedon a corresponding scale in FIG. 2 by B In case b an order has beengiven to retard or reverse the speed of the motor. First the converteris retarded towards inverter operation corresponding to an increasingcontrol angle a. The current in the circuit falls rapidly towards zeroafter which the converter is blocked and shifts polarity. The converteris then unblocked, see FIG. 1c and 20, which according to previouslyknown methods, took place at full inverter operation, that is, with acontrol angle (1 However, according to the invention the armaturevoltage of the motor is sensed and the current regulator obtains aninput magnitude corresponding to a higher voltage E, and control angle04 Only a small reduction of the control angle is then necessary beforethe converter voltage E, passes the motor voltage and the current startsto flow in the direction of the arrow. The converter operates asinverter and power is fed from the motor to the converter so that thespeed and thus the motor voltage E fall rapidly. If the motor is to bereversed this process continues since the control angle a and thus E,are reduced in time with the motor voltage E, and gradually theconverter switches over to rectifier operation, the motor reverses andthe speed increases in the opposite direction until the desired speed isachieved.

If the speed is only to be reduced, when the lower speed has beenreached the converter will again be retarded towards inverter operationso that the current becomes zero again. The converter is blocked andshifts polarity again and when it is again unblocked, case d is arrivedat. According to previous methods the converter would even now beunblocked at full inverter operation, that is, control angle a afterwhich the converter would be advanced towards rectifier operation untilits voltage E, exceeded the motor voltage E According to the invention,however, the current regulator is again given such an input voltagesignal that the converter is unblocked with a control angle a,corresponding to rectifier operation and a voltage E, which is onlyslightly less than the motor voltage E After this only a tiny reductionof the control angle a is required for the converter voltage E, toexceed the motor voltage E so that power is fed from the converter tothe motor by current in the direction of the arrow. During a reductionin speed, therefore, two shifts of polarity are required of theconverter whereas in reversing only one shift of polarity is required.

FIG. 3 shows a converter connection according to the invention where aconverter 8, fed from an alternating current network I, over thecontacts in two pole switches 10, 11, feeds a DC motor 9. This isprovided with a tachometer-generator 12, the voltage of which isconnected to a summation member 14 which forms the input circuit of acurrent reference emitter which is an amplifier 15. A desired valueemitter for the motor speed is also connected to the member 14 in theform of a potentiometer 13, the center point being earthed. Thepotentiometer 13 and the tachometer-generator 12 are arranged inrelation to each other and to the control system as a whole so that acertain direction of rotation of the motor corresponds to negativeoutput voltage from the tachometer-generator and positive voltage fromthe potentiometer 13.

From the current reference emitter 15 a desired value is obtained forthe direct current which corresponds to the difference between thedesired value and the actual value of the speed. The desired value ofthe current is connected to a summation member 18 over one or twoswitching members 2, 3, and a polarityreverser 4 is arranged in serieswith 3 so that the desired value during stationary conditions is alwaysconnected with the same polarity to 18. The switching members 2 and 3are controlled by individual contacts in one of the pole shifters 10 or11, respectively and have the character of electronic make-contactrelays. A signal is also connected to the member 18 which corresponds tothe actual value of the direct current and which can be obtained from acurrent meter, for example a measuring transductor 16 in a conductorbetween the converter and the pole shifting contacts.

The output from the member 18 is connected to a current regulator 7 ofknown type which delivers a control voltage E, corresponding to thedesired converter voltage, see FIGS. 1 and 2.

The desired value of the current from 15 as well as the actual valuefrom 16 are further connected to a control member 5 for the poleshifters 10 and 11. The armature voltage of the motor is taken out andconnected through a voltage reduction circuit 17 (DC transformer,Hall-generator, voltage divider or the like) to a voltage adjuster 6.The control member 5, voltage adjuster 6 and current regulator 7 areshown more in detail in FIG. 4 and will be described in connection withthe function of the system in FIG. 3.

When the motor is started in the forward direction, a certain positivevoltage is set on the potentiometer 13. Since the motor is standingstill, the tachometergenerator delivers no voltage so that the currentreference emitter 15 is only influenced by the potentiometer voltage andemits a negative desired value which is connected to the control member5 through the terminal IR and to the switching members 2 and 3 whichare, however, blocked for the moment.

In the control member 5 the negative desired value, also designated IR,is supplied to a discriminator 501 which only emits an output signalwhen IR is positive. In this case, therefore, the output signal will bezero from 501 so that the signal inversing gate 506 gives an outputsignal over the switching member 504, the flipflop device 505 moves tozero" position and delivers an output signal which is supplied to theAnd-gate 510, directly as well as through the switching member 508 whichis controlled by the signal switching gate 514.

This has for the moment no input signal and its output signal thereforekeeps the member 508 open, the flipflop 509 moves to zero position sothat the And-gate 510 receives signals at both inputs and gives astarting signal to the forward relay in FIG. 3, the contacts of whichclose. The signal from 510 will be supplied through the gates 515 and512 and the member 513, as described later, to the signal inverter 514,the output signal of which then disappears and 508 is blocked. Nothinghappens, however, since the flip-flops 505 and 509 are already in zeroposition.

When the forward relay 10 closes, the member 2 in FIG. 3 is opened, thecurrent reference IR is supplied through the member 18 to the currentregulator 7 which delivers positive control voltage E, to the converter8 which delivers current to the motor 9 through the contacts in theforward direction and the motor starts in the forward direction andincreases its speed until the signal from the tachometer-generator 12agrees with that set on the potentiometer 13. In the same way thecurrent in the converter and the motor increases and its value IS,measured by the transductor 16, is supplied to the members 18, 5 and atthe desired speed the members 14, 15, 18 and 7 are balanced. The actualvalue of the converter current, as mentioned, is supplied to the member5 through the terminal IS which leads to a discriminator 502 which, like501,.

only gives an output signal for positive input values. The circuit is soconstructed that the signal from the measuring transductor 16 can onlybe positive so that 502 delivers an output signal to 503, 504 and 512.512 is an Or-gate which already has an input signal from 515. 503 and504 are blocked by the input signal, which gives no alteration, however,since the flip-flop 505 is already in its zero position. The motor thuscontinues its operation undisturbed. The case corresponds to FIGS. 1aand 2a.

When the speed is retarded either to a lower speed or for reversal ofthe motor, the potentiometer 13 is set at a lower or negative value,respectively, so that the tachometer voltage will dominate in the member14 and the desired value of the current, that is, the output from 15,changes sign and becomes positive. The discriminator 501 in FIG. 4 willthen emit an output signal but since the members 503 and 504 areblocked, this has no immediate consequences.

On the other hand, the altered polarity of IR will affect the members 2,18 and 7 so that the control voltage E, to the converter 8 tends towardsa negative value, and the converter is then retarded towards inverteroperation corresponding to FIGS. lb and 2b. The main current thendecreases and IS thus rapidly becomes zero so that the output signalfrom the discriminator 502 disappears. The members 503 and 504 are thenopened, the latter being without an input signal from 506. On the otherhand, due to the signal from 503 the flip-flop 505 will move tol-position sothat the lower input signal to the gate 510 disappears andthe forwards relay 10 is disconnected. The converter and motor are thusdisconnected. The member 2 blocks so that the current reference IR to 18disappears.

The gate 515 and thus gate 512 will now be without signals, and theinput signal to the switch 514 thus also disappears. However, this doesnot take place immediately. A delay member 513 maintains the signal to514 a moment so that the members 507 and 508 remain blocked. Thedisappearance of the output signal from 515 causes a switching member atthe input of the summation member 18 in FIG. 4 to become conducting, sothat a positive voltage over a resistor 74 is supplied to the currentregulator 7. This comprises an amplifier 71 with feedback capacitor 72and the positive input voltage from 74 through 75 and 18 causes anegative output voltage from 71 and thus full retardation of theconverter. This retardation, however, is limited corresponding to themaximum permitted control angle (oz in FIG. 20), that is, inverteroperation with the help of a limit value emitter in the form of apotentiometer 73 fed from a positive voltage source and connected to 18over a diode 78, a voltage divider 76, 77 and a switching member 65controlled by the signal from 512. The signal E therefore, cannot bemore negative than the voltage from 73 determines.

As mentioned earlier, the armature voltage is taken out from the motor 9and a reduced voltage E is fed through the member 17 proportional to thearmature voltage to the device 6. When the motor is running in theforward direction E is regarded as positive. The flip-flop 505 is inl-position and emits signals to the upper input of the And-gate 511 andto the switching member 61 so that the signal from 17 through 61 and aresistance combination 63a, b and 67 is supplied to the switching member64 controlled from the gate 512. The signal from this gate 512 is zeroso that 64 is open while 65 is closed. This means that the limit valueemitter 73 is replaced by the voltage adjusting device 6 and theregulator 7 will receive an input voltage corresponding to the controlangle a in FIG. 2c and a converter voltage which is less than thearmature voltage of the motor only by a quantity which is determined by63a, 63b and 67 and which substantially corresponds to the ripple in thedirect voltage of the converter. This voltage is stored in the feedbackcapacitor 72 in the regulator 7.

When the delay member 513 then interrupts the input signal to 514, themember 507 is opened so that the flip-flop 509 moves to l-position. Thegate 511 thus receives a double input signal and switches the reverserelay 11 to its on-position. The current reference IR is then connectedover the polarity-reverser 4 and member 3 to 18 and thus to theregulator 7. At the same time the gate 515 receives an input signal anddelivers an output signal to 512 and 75. The retardation circuit 74, 75is thus blocked and switching takes place from 6 to 73. The signal from6, however, remains in the capacitor 72. The converter 8 will thus beconnected to the motor in a control position with the control angle :1according to FIGS. 1c and 20. Since there is no switching on delay in513, the signal from 515 through 512 and 513 causes the output signalfrom 514 to disappear so that 507 and 508 are blocked. The flipflop 509remains in its l-position, however.

The current reference IR is positive but is reversed in thepolarity-reverser 4 so that the converter tends towards a decreasingcontrol angle. The direct current will thus very quickly become equal tothe current set corresponding to IR, but the converter operates as aninverter and power is fed from the motor to the converter so that themotor is braked. The direct current then decreases and the regulatordecreases the control angle and, when the motor is reversed, the processcontinues with substantially constant direct current but decreasingcontrol angle. When this passes 90 the converter is transferred torectifier operation, the motor is braked completely and alters itsdirection of rotation. It is then driven in reverse until the newreverse speed and thus balance in the control chain l4, l5, 4, 3, 18, 7,is achieved. When the current increases IS emits a signal through 502 sothat 503 and 504 are blocked but the flip-flops 505 and 509 remain inl-position.

If the speed is only to be reduced the process described above will beinterrupted when the lower speed is passed. The signal then changespolarity in the chain 14, 15, 4, 3, l8, 7, and the converter is againretarded and the direct current and thus IS becomes zero.

When IR shifts polarity and becomes negative, the signal from 501disappears and 506 emits a signal. When IS is zero 504 opens and theflip-flop 505 moves to zero position. The reverse relay 11 drops andmember is blocked. The signal from 515 and from 512 disappears. Theretardation 74, 75 is connected and the limiting device 73 is replacedby the member 6 which, this time through the polarity-reverser 66 andswitching member 62, sets a new signal in the regulator 7, correspondingto control angle a, according to FIGS. 1d and 2d. After a while 513interrupts the signal to 514 so that the zero signal from the flip-flop505 is supplied to the gate 510 directly as well as over 508 and theflipflop 509 which moves to zero position. 515 and 512 again emitsignals so that the retardation 74, 75 disappears and 6 is replaced by73. 507 and 508 are blocked.

The negative current reference IR is connected over the member 2 andgives positive control voltage E from 7 with the output angle or, andthe converter, which has now once more been connected over the forwardrelay 10, is advanced until its direct current increases until IS isequal to IR and the speed is fixed at a tachometer voltage correspondingto that set on the potentiometer.

Whereas previously after a switching it was always necessary to startthe converter with full retardation, that is, maximum control angle adictated by the limit value emitter 73, according to the invention theconverter is always started with a control angle (a 01 corresponding toa converter voltage E, which is slightly below the motor voltage E inquestion. The subsequent advance thus means that the converter becomesalmost immediately current carrying and excellent continuity in theswitching is obtained. Technically, therefore, it is the voltageadjusting device 6 which is novel according to the invention.

It is also seen that the magnitude of the gap which is avoided dependson the difference between the motor voltage E and the voltagecorresponding to full retardation of the inverter, in other words, thedifference between the control angle a or a and the maximum controlangle :1 The greatest time gain is obtained according to FIGS. 1d and 2dwhich is much more impor tant since in this case there are twoswitchings.

In FIG. 3 the polarity shifter is shown as a contact switch. In systemswhere the demand for speed is such that the voltage adjuster describedabove is used, it is however preferred to replace the contact switch bya valve switch having reverse-parallel-connected thyristor valves, sothat a more rapid switch-over is obtained. The relays l0 and 11 in FIG.3 are then replaced by control devices for the thyristor valves. Theprinciple is illustrated in FIG. which shows the converter 8 and themotor 9 connected through two reverse-parallei-connected sets ofthyristors 10a, b and 11a, b, controlled by the And-gates 510 and 511,respectively.

It should also be pointed out that the blocking of the converter 8according to FIGS. 3, 4 and 5 is done by disconnecting the currentreference IR over 2 or 3 and connecting the retarding signal from 74,75.

A system which in its function is equivalent to that in FIG. 5 isobtained if the converter 8 is replaced by tworeverse-parallel-connected converters 8a and b, as shown in FIG. 6.These may be provided with a common control pulse generator 8c which iscontrolled by the current regulator 7. Connection and disconnection ofthe converter is then done by connecting and disconnecting the controlpulses by means of the switching members 10d and 11d controlled by thecontrol member 5 in the same way as the switches 10 and 11 in I FIG. 3.In this case it is seen that upon a switching, one

of the converters is blocked by disconnecting the pulse generator 80,.after which the other converter is deblocked when the control pulsegenerator is connected to it.

We claim:

1. In combination with an AC. network and a DC. load, a reversibleconverter arrangement comprising a rectifier andfeeding the DC. loadfrom the AC. network, a current regulator for said converter; means tosupply to the input side of the current regulator a signal representingthe difference between the actual value of the direct current of theconverter and a desired DC value in the converter; said currentregulator including means responsive to the input signal to produce apredetermined direct voltage in the rectifier output, and means forswitching the polarity. of the converter depending on the polarity ofthe desired value of the current; said means on the input side of thecurrent regulator including means responsive to a shifting of thepolarity of said desired D.C. value to retard the converter towardsinverter operation before shifting of the polarity of the converter andconsequent unblocking and advancing towards rectifier operation, theimprovement in which the converter comprises voltage limiting means forsaid converter, said voltage limiting means including means operativelyconnected to said polarity shifting responsive means and responsive to ashifting of the polarity of said desired D.C. value, said voltagelimiting means comprising a member to sense the voltage over said D.C.load, said voltage limiting means including means to limit the voltageof the converter to a value which is a little less than the voltage oversaid D.C. load.

2. Combination according to claim 1, in which said amount is less than,but substantially equal to, the amplitude of the D.C. ripple of theconverter.

3. Combination according to claim 1, wherein said converter arrangementcomprises two reverse-parallelconnected converters, a common controlpulse device, separate members connecting the control pulse device tothe rectifiers of the converters, and means respon sive to the polarityof the desired value of the current to block said connecting membersalternatively.

value, blocking of the converterwith consequent shifting of polarity andunblocking thereof.

5. Combination according to claim 4, wherein said converter arrangementcomprises two reverse-parallelconnected converters, said blockingdevice, depending on the polarity of the desired value, blocking one orthe other of said converters.

1. In combination with an A.C. network and a D.C. load, a reversibleconverter arrangement comprising a rectifier and feeding the D.C. loadfrom the A.C. network, a current regulator for said converter; means tosupply to the input side of the current regulator a signal representingthe difference between the actual value of the direct current of theconverter and a desired DC value in the converter; said currentregulator including means responsive to the input signal to produce apredetermined direct voltage in the rectifier output, and means forswitching the polarity of the converter depending on the polarity of thedesirEd value of the current; said means on the input side of thecurrent regulator including means responsive to a shifting of thepolarity of said desired D.C. value to retard the converter towardsinverter operation before shifting of the polarity of the converter andconsequent unblocking and advancing towards rectifier operation, theimprovement in which the converter comprises voltage limiting means forsaid converter, said voltage limiting means including means operativelyconnected to said polarity shifting responsive means and responsive to ashifting of the polarity of said desired D.C. value, said voltagelimiting means comprising a member to sense the voltage over said D.C.load, said voltage limiting means including means to limit the voltageof the converter to a value which is a little less than the voltage oversaid D.C. load.
 2. Combination according to claim 1, in which saidamount is less than, but substantially equal to, the amplitude of theD.C. ripple of the converter.
 3. Combination according to claim 1,wherein said converter arrangement comprises tworeverse-parallel-connected converters, a common control pulse device,separate members connecting the control pulse device to the rectifiersof the converters, and means responsive to the polarity of the desiredvalue of the current to block said connecting members alternatively. 4.Combination according to claim 1, in which the converter arrangementcomprises a blocking device connected to the input side of the currentregulator and influenced by the real value and the desired value of thecurrent, and including means responsive to an alteration in polarity ofthe desired value to cause first a retarding of the converter towardsinverter operation, and, when the real value has fallen to a certain lowvalue, blocking of the converter with consequent shifting of polarityand unblocking thereof.
 5. Combination according to claim 4, whereinsaid converter arrangement comprises two reverse-parallel-connectedconverters, said blocking device, depending on the polarity of thedesired value, blocking one or the other of said converters.